1. Field of the Invention
The present invention relates generally to a closed-loop transmit diversity system using a plurality of transmission antennas, and in particular, to a method and apparatus for adaptively determining weights of respective antennas according to a condition of channels.
2. Description of the Related Art
Transmit diversity is an effective technology for relieving degradation of a signal due to fading in a wireless communication system. In the transmit diversity technology, a transmitter transmits desired transmission data via a plurality of transmission antennas so that a receiver can receive data in a stable manner.
FIG. 1 is a block diagram illustrating a simplified baseband structure of a closed-loop transmit diversity system having M transmission antennas. Here, “closed-loop” means that a transmitter uses information fed back from a receiver in transmitting data via a plurality of transmission antennas.
As illustrated, in a transmitter 100, a modulator 110 receives a bit stream including desired transmission information and generates a data symbol d having symbol energy Es according to a predetermined modulation scheme. M multipliers 120, 122 and 124 multiply the data symbol d by weights w1, w2, . . . , WM assigned to corresponding antennas, and provide their outputs to M transmission antennas 130, 132 and 134. The M transmission antennas 130, 132 and 134 radiate the weighted data symbols dwm to the air.
In a receiver 140, a reception antenna 150 receives a radio signal from the transmitter 100. An adder 160 indicates that an additive white Gaussian noise (AWGN) is added while the receiver 140 receives the radio signal. The radio signal includes a previously known pilot together with the data transmitted by the transmitter 100. A channel estimator 170 calculates channel gains h1, h2, . . . , hM indicating channel impulse response characteristics of the receiver 140, from the M transmission antennas 130, 132 and 134, using the pilot. The data excluding the pilot is provided to a demodulator 180 so as to restore its original information.
An antenna weight determiner 190 calculates antenna weights w1, w2, . . . , WM using the calculated channel gains h1, h2, . . . , hM, and the calculated antenna weighs are transmitted to the transmitter 100 over a feedback channel 10, using undepicted transmission elements.
In the transmit diversity system constructed in this way, a procedure for calculating antenna weights for the transmission antennas 130, 132 and 134 by the antenna weight determiner 190 is performed as follows. First, a received signal in a receiver is expressed as shown in Equation (1):
                    r        =                                            ∑                              m                =                1                            M                        ⁢                                                  ⁢                                                            E                  s                                            ⁢                              dw                m                            ⁢                              h                m                                              +          n                                    (        1        )            Here, r is a received signal, m is a transmission antenna index, M is the number of transmission antennas, Es is symbol energy used in a transmitter, and d is a data symbol. When d is a binary data symbol, Es is equal to bit energy Eb. In addition, wm is a weight assigned to an mth antenna, hm is a channel characteristic from an mth transmission antenna to the receiver, and n is a channel noise.
If a data symbol d is a binary data symbol having the same probability of d=+1 and d=−1, an instantaneous signal-to-noise ratio (SNR) of a received signal is given as shown in Equation (2):
                    γ        =                                            E              s                        ⁢                                                                                                ∑                                          m                      =                      1                                        M                                    ⁢                                                            w                      m                                        ⁢                                          h                      m                                                                                                  2                                            N            o                                              (        2        )            
Here, No is AWGN energy. In order to maximize SNR, antenna weights are determined in accordance with Equation (3) below (see R. T. Derryberry, S. D. Gray, D. M. Ionescu, G. Mandyam, and B. Raghothaman, “Transmit Diversity in 3G CDMA System,” IEEE Commun. Mag., pp. 68-75, April 2002).
                              w          m                =                              h            m            *                                                              ∑                                  m                  =                  1                                M                            ⁢                                                                                      h                    m                                                                    2                                                                        (        3        )            
Here, hm* means a complex conjugate of the channel characteristic.
FIG. 2 is a block diagram illustrating a detailed structure of the antenna weight determiner 190 according to the prior art. As illustrated, complex conjugate generators 191-1 to 191-M calculate conjugates of channel gains h1, . . . , hM, and power gain acquisition sections 192-1 to 192-M calculate channel power gains by squaring absolute values of the channel gains. A summer 193 sums up the channel power gains, a square root generator 194 calculates a square root of the summed channel power gain, and a reciprocal generator 195 calculates a reciprocal of the square root. Multipliers 196-1 to 196-M calculate weights w1, . . . , wM by multiplying the conjugates of the channel gains by the reciprocal of the square root.
In the conventional weight determining method described above, power consumed during transmission of data symbols is normalized so that the power is always constant. That is, during transmission of each data symbol, the sum of power gains of antenna weights always becomes 1, so the SNR of a received signal is maximized by distributing transmission power according to spaces (antennas). This conventional technology considers only a space variation characteristic among time and space (antenna) variation characteristics of channels. Thus, the conventional technology fails to minimize an average bit error rate (BER) serving as an important criterion for performance evaluation.